(a) Field of the Invention
The present invention relates to a light emitting display, and more particularly to a method of compensating for the variance in the threshold voltage of the driving transistor among the pixel circuits of the light emitting display.
(b) Description of the Related Art
In general, an organic light emitting diode (OLED) display, which is a light emitting display for displaying images using the electroluminescence of an organic material, displays images by driving N×M organic light emission cells arranged in a matrix on a voltage basis or a current basis.
An organic light emission cell, which is also called an organic light emitting diode (OLED) because the cell has diode characteristics, has a multi-layered structure including an anode layer which may be made of indium tin oxide (ITO), an organic thin film layer, and a cathode layer which may be made of metal. The organic thin film also has a multi-layered structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL). The organic thin film further includes a separate electron injecting layer (EIL) and a separate hole injecting layer (HIL). Therefore in one embodiment, an OLED display panel may be formed by arranging organic light emission cells in an N×M matrix.
Methods for driving the OLED display panel are generally classified as either a passive matrix method or an active matrix method using thin film transistors (TFTs). In the passive matrix method, anodes are perpendicular to cathodes and lines are selected and driven, while in the active matrix method, TFTs are coupled to respective ITO pixel electrodes and are driven by voltages maintained by capacitance of capacitors coupled to gates of the TFTs.
FIG. 1 is an equivalent circuit diagram of a pixel circuit employing a conventional active matrix method.
As shown in FIG. 1, the pixel circuit includes an OLED element (OLED), two transistors including a switching transistor SM and a driving transistor DM, and a capacitor Cst. Each of the two transistors SM and DM is a PMOS transistor.
The switching transistor SM has a gate electrode coupled to a scan line Sn, a source electrode coupled to a data line Dm, and a drain electrode coupled to one end of the capacitor Cst and a gate electrode of the driving transistor DM. The other end of the capacitor Cst is coupled to an operation voltage VDD. The driving transistor DM has a source electrode coupled to the operation voltage VDD and a drain electrode coupled to a pixel electrode of the OLED element (OLED). The OLED element (OLED) has a cathode coupled to a reference voltage Vss and emits light under application of current through the driving transistor DM. In this embodiment, the reference voltage Vss coupled to the cathode of the OLED element (OLED) is a voltage lower than the operation voltage VDD. For example, the reference voltage Vss may be a ground voltage.
In operation of the pixel circuit as configured above, when a select signal is applied to the scan line Sn and the switching transistor SM is then turned on, a data voltage is applied to the one end of the capacitor Cst and the gate electrode of the driving transistor DM. Accordingly, a gate-source voltage VGS of the driving transistor DM is maintained for a certain time by the capacitor Cst. The driving transistor DM applies current IOLED corresponding to the gate-source voltage VGS to the pixel electrode of the OLED element (OLED), causing the OLED element (OLED) to emit light. At this time, the current IOLED flowing through the OLED element (OLED) is expressed by the following Equation 1.
                                                                        I                OLED                            =                                                β                  2                                ⁢                                                      (                                                                  V                        GS                                            -                                              V                        TH                                                              )                                    2                                                                                                        =                                                β                  2                                ⁢                                                      (                                                                  V                        DD                                            -                                              V                        DATA                                            -                                                                                                V                          TH                                                                                                              )                                    2                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
As can be seen from Equation 1, when a high data voltage VDATA is applied to the gate electrode of the driving transistor DM, the gate-source voltage VGS of the driving transistor DM is lowered at which point a small amount of current IOLED is applied to the pixel electrode resulting in a low light emission from the OLED element (OLED), and hence low gray scales of the OLED display panel. In contrast, when a low data voltage VDATA is applied to the gate electrode of the driving transistor DM, the gate-source voltage VGS of the driving transistor DM is raised at which point a large amount of current IOLED is applied to the pixel electrode, resulting in a high light emission from the OLED element (OLED), and hence high gray scales of the OLED display panel. In this way, a level of the data voltage applied to the pixel circuit may be determined based on an image data signal to be displayed.
However, as can be seen from Equation 1, in the pixel circuit as mentioned above, the current IOLED depends on a threshold voltage Vth of the driving transistor DM. Therefore, a difficulty may arise in accurately displaying images due to the different threshold voltages of the driving transistor DM for different pixels.